The goal of this project is to determine the mechanism of action of hormone-regulated transcription factors, called nuclear receptors (NR) at the atomic level. Hormonal molecules such as glucocorticoids, retinoids, thyroid and vitamin-derived hormones exert their effects by regulating the transcription of hormone-responsive target genes within the nucleus of cells. These hormones function by directly binding to and modulating the activity of NRs. These hormone-regulated proteins direct almost every aspect of human physiology and improper function can lead to several disease states such as prostate and breast cancer, diabetes, obesity, heart disease, osteoporosis, and processes associated with aging. NRs often function as heterodimers such as the thyroid receptor: retinoid X receptor (TR:RXR) and the constitutive androstane receptor (CAR:RXR) complexes where TR, CAR and RXR can each recognize specific hormonal signals. Despite the wealth of data on the genetics and cellular localization of NRs, there is relatively little known of the precise molecular mechanisms of regulation of these proteins. The ligand binding domain (LBD) of these receptors is central to the allostery that is essential for NR transactivation. Using a combination of biophysical tools and cell- based transcription assays this project will determine the how hormone-binding and point mutations at distant sites can affect the recruitment of co-regulatory proteins and how the two hormone-binding sites communicate to regulate the transcriptional activity of heterodimeric TR:RXR and CAR:RXR. Structures will be determined by X-ray crystallography to guide biophysical analyses of conformational movements and transcriptional activity. An important consequence of these studies on the structure and dynamics of NRs is the potential for developing more effective therapies.